Magma wipe-out A mass extinction event 200 million years ago that wiped out half of all species on Earth was caused by volcanic activity, a new study says.

The finding reported today in the journal Science, provides the tightest link yet between the release of huge amounts of volatile gases during a series of mega-eruptions and a global die-off of marine and terrestrial species known as the end-Triassic-extinction.

The researchers say these eruptions may have triggered climate changes so sudden that many creatures were unable to adapt and add the situation is analagous to the pace of human-induced climate change today.

"This set the stage for dinosaurs to dominate Earth for the next 135 million years, until they too were wiped out by an extinction event," says lead author Dr Terrence Blackburn, then at the Massachusetts Institute of Technology.

Scientists had previously connected the timing of volcanic mega-eruptions with several mass extinction events, but these estimates have a margin of error of one to three million years.

Blackburn and colleagues developed a significantly more precise date with a margin of error of only a few thousand years (the blink of an eye in geologic time) by analysing samples of basalts from a formation called the Central Atlantic Magmatic Province.

Supercontinent

This basalt formed through the separation of North America and northern Africa during the rifting of the Pangean supercontinent to form the Atlantic Ocean basin.

"As the sea floor spread, the massive outpouring of basaltic magma released gases including carbon dioxide into the atmosphere and acidifying the oceans," says Blackburn.

"Over 600,000 years more than three million cubic kilometres of basaltic lava was produced in four pulses."

Blackburn and colleagues used the decay of uranium isotopes contained in zircon crystals embedded in basalt samples to get exact dates for the eruption events. The samples were collected from New Jersey, Nova Scotia and Morocco.

"Zircon crystals form in solidifying magma and contain uranium which decays at a set rate that we can measure precisely," says Blackburn.

The first of these pulses, found in samples from Morocco, produced more than a million cubic kilometres of magma and coincides exactly with the start of the end-Triassic-extinction 201,564,000 years ago.

This date is supported by changes in a layer of sedimentary mineral grains linked to one of Earth's periodic magnetic pole reversals known as the E23r event.

Evidence of the reversal is consistently found in sedimentary rock located just below the extinction event, making it a convenient marker.

The samples from Nova Scotia indicate eruptions occurred about 3000 years later, while those from New Jersey point to an eruption about 13,000 years after the Morroccan event.

Sediments below that time contain pollen, spores and other fossils characteristic of the Triassic era, while in those sediments above the fossils disappear.

The researchers say that among the creatures that disappeared were early crocodilians, tree lizards and many broad-leaved plants.

Lessons for today

According to Blackburn, the initial mass extinction event lasted no more than 20,000 years.

This conclusion was reached by correlating the precisely dated basalts with surrounding sedimentary layers generated by temperature changes that affected lake water levels.

These layers are the result of a 20,000-year cyclic change in the orientation of the Earth's axis toward the sun.

The study shows the extinction event occurred in just one layer - meaning the event took 20,000 years at the most.

Blackburn says there are lessons to be learned from this study.

"In some ways, the end-Triassic-extinction is analogous to what's happening today," says Blackburn.

"It's operating on a similar time scale.

"So we can gain an insight on the future impact of increasing atmospheric carbon-dioxide levels on global temperatures, ocean acidity and life, by studying the geologic record."